1. Field of the invention
The present invention relates, in general, to refrigerators.
More particularly, the invention relates to a refrigerator with a defrost override system for automatically deactivating the defrost device in response to heating of the evaporator for longer than a preset time.
2. Description of the Prior Art
As is well known, a refrigerator typically is provided with a defrosting system for removing the frost accumulated on the evaporator. A defrosting system of such a well known refrigerator normally includes a defrost timer for counting the cumulative running time of the compressor in order to start the defrosting operation, a defrost heater for thawing the frost accumulated on the evaporator and a defrost sensor for detecting the temperature of the evaporator in order to stop the defrosting operation.
The defrosting operation is controlled and carried out periodically by a microcomputer of a control circuit. When the cumulative compressors running time counted by the defrost timer reaches a prescribed level, such as, e.g., thirteen hours, the compressor is prevented from operating, and the defrost heater is activated. While the defrost heater is on, the frost on the evaporator is gradually thawed by radiant heat from the defrost heater, and in accordance with the thawing of the frost, the temperature of the evaporator gradually rises. In this period, the defrost sensor detects the temperature of the evaporator. When the temperature of the evaporator detected by the defrost sensor reaches a prescribed value, such as, e.g., 13 .degree. C. the defrost heater is deenergized, and the compressor is returned to an operational state.
As is evident, it is the most important function of a refrigerator to maintain the temperature of the food stored therein at an appropriate level such as, e.g. --18.degree. C. Even during the defrosting operation, it is necessary to maintain the temperature of the food below the prescribed level. However this defrosting system of the conventional refrigerator, when the defrost sensor experiences some defect, such as. e.g., shorting or breaking, the defrost operation does not stop. This is because the defrost sensor cannot detect when the temperature of the evaporator heated by the defrost heater has reached the prescribed upper limit value, as described above. Therefore, the defrost heater continues to operate, the compressor operation continues to be prevented, and the temperature of the food continues to rise higher and higher until a thermal fuse in the control circuit opens to prevent the refrigerator from catching fire.
Also when the defrost heater has some defect, such as, e.g.,a broken wire or glass, the defrost operation may not stop. This is because the defrost heater cannot generate radiant heat to thaw the frost on the evaporator, and the temperature of the evaporator does not reach the prescribed value described above. Therefore, the compressor continues to be interrupted, and the temperature of food continues to rise higher and higher until the temperature of the evaporator naturally, without being heated by the defrost heater, reaches the prescribed value, such as, e.g., 13.degree. C.
Particularly in winter, the compressor operation is interrupted almost continuously because the temperature of the evaporator practically never reaches the prescribed value, as described above, As a result, food such as ice cream may be melted, and in the worst condition, food such as eggs may rot.
In order to solve the problem mentioned above, a defrost warning system has been developed for informing a user that there is some defect in the defrosting operation. An example of such a defrost warning system is disclosed in Japanese Utility Model Publication No. 52-37882, filed on Sept. 26, 1975 in the name of Minoru Igarashi, etc. In Japanese Utility Model Publication No. 37882, a warning timer for(counting the defrost time is provided with the control circuit. When the count of the warning timer reaches a prescribed value before the completion of the defrosting operation, an audible warning is given to the user of the refrigerator. By this warning the user is made aware that the defrosting operation was not carried out correctly, and the user can manually stop the defrosting operation. However even with this prior art system, the defrosting operation could be continued for an extraordinarily long time, and the temperature of the food could rise to such a high degree that the food is melted or turns rotten.
The prior art system is based on the idea that the user will manually stop the defrosting operation. Therefore, when in spite of the warning the user of the refrigerator is unaware that the defrosting operation is wrong, such as, e.g., when the user is not within hearing distance of the warning system the prior art system does not prevent overheating of the refrigerator. Accordingly, because the warning may be generated in vain, and since the defrosting operation may not be stopped by a user, the defrosting operation can continue t.COPYRGT.be carried out for an extraordinarily long time, and the temperature of the food may rise to an extraordinarily high degree.